The invention relates to a process for the recovery and reuse of cobalt and or manganese components of catalysts used in the oxidation of alkyl aromatics in acetic acid solution with atmospheric oxygen, such recovery being performed after the separation of insoluble target products or prior to the separation of soluble target products, by the precipitation and separation of the cobalt and/or manganese present in the form of soluble compounds in the mother liquor reaction mixture, by converting them to insoluble cobalt and/or manganese oxalate dihydrate, so that they will be usable in a process for the preparation of terephthalic acid by means of xylene oxidation in acetic acid solution catalyzed either by cobalt and/or manganese bromide, or by other cobalt and/or manganese salts in the simultaneous presence of bromides, as is described, for example, in DE AS No. 1,081,445.
By such methods, however, still other aromatic mononuclear or polynuclear mono-, di- or polycarboxylic acids can be prepared, the aromatics also being additionally to bear one or more identical or different oxidation-resistant groups as substituents. Examples that can be mentioned here are the oxidations of chloro, nitro or tert. butyl toluene to the correspondingly substituted benzoic acids, or dimethylnaphthaline to naphthalinedicarboxylic acid, or of ditolylsulfone to sulfonyldibenzoic acid. In some of these processes, in addition to cobalt or manganese salts and bromides, still other salts are added as cocatalysts or organic promotors.
The recovery and reuse of cobalt and/or manganese salts from the above-named oxidation processes constitute an important problem, but at the same time a difficult one. In a number of processes for the recovery of cobalt and/or manganese catalysts, these heavy metal components are isolated as carbonates from the mother liquor after separation of the target product, in accordance, for example, with DE OS No. 2,131,470, DE OS No. 2,260,491 and DE OS No. 2,419,323. For this purpose the mother liquor must first be concentrated and the concentrate then extracted with water. This process is technically very complex and not without problems. The extraction of the often tarry concentrate, consisting sometimes of two phases, can present considerable difficulty. Together with the cobalt and/or manganese salts, other heavy metals contained as impurities in the mother liquor and hence also in the concentrate may also become precipitated, and then will interfere with the subsequent oxidation when the catalyst is reused.
In JA OS No. 97,593/76 there is described a process for the recovery of cobalt and/or manganese salts from oxidation processes, in which, after the target product has been separated, the cobalt and/or manganese are precipitated from the acetic acid mother liquor by the addition of oxalic acid thereby converting them to oxalates of low solubility, and other heavy metal oxalates can be removed by washing the precipitate. However, the recycling of the virtually insoluble cobalt and/or manganese oxalates to the oxidation process is problematical.
It is the object of the invention to recycle into the oxidation process the cobalt and/or manganese catalysts which have been isolated in the form of very poorly soluble oxalates from acetic acid mother liquors originating in such oxidation processes. No drying process is to be used. This is to be accomplished by restoring the solubility of these catalysts in acetic acid after replacing any catalyst losses that might have been incurred, without thereby impairing the action of the recycled catalyst or adversely affecting the oxidation process by causing secondary reactions, or impairing the yield and purity of the target products. The isolation and recycling of the catalysts is to be repeatable any desired number of times. In other words, the heavy metal catalyst should be able to be precipitated again from the mother liquor that originates from an oxidation process performed with recycled catalyst, and again isolated, and again returned to another cycle of the process.